As an optical frequency control device, the optical single-side-band (SSB) modulator has already known in the art. An optical SSB modulator is the optical modulator which can obtain the output light shifted by the frequency of a modulation signal (S. Shimotsu, S. Oikawa, T. Saitou, N. Mitsugi, K. Kubodera, T. Kawanishi and M. Izutsu, “Single Side-Band Modulation Performance of a LiNbO3 Integrated Modulator Consisting of Four-Phase Modulator Wavegate,” Institute of Electrical and Electronics Engineers Photon. Tech. Lett., Vol. 13, and 364-366 Reference (2001) (non-patent document 1 and 2).
The frequency control technology using an optical SSB modulator has a feature that using the technology precision becomes relatively high and stability becomes excellent. However, the speed for sweeping the optical frequency using an optical SSB modulator is restricted by the sweep speed of the modulation signal which is electrical signal. For example, when the electrical signal frequency sweep unit with a wide frequency range was used, the sweep rate was limited. On the other hand, although the FM modulator can carry out the sweep of the frequency at comparatively high speed, its frequency transition width was as narrow as several 100 MHz or less. Therefore, since the signal control which has a high sweep rate and a large frequency range could not be attained, the frequency control technology of an optical SSB modulator also had the problem that it was incompatible in them.